The usual procedure in solar concentrators is to use a standard type Fresnel lens designed for point focus and to place the target inside of focus to obtain the desired concentration ratio and to slightly modify the lens facet angles to improve the uniformity of the intensity. There are three problems with this design approach which are particularly noticeable for photovoltaic concentrator arrays where one wants to use minimum size cells.
First, the image of the sun projected on the target has poorly defined boundaries because (assuming the lens is circular) they are formed by the rays mainly passing through the edge portion of the lens. The outer rays are more poorly defined at the target plane than the central rays due to the greater spread from dispersion and facet imperfections. These outer rays also have a longer path length giving rise to a larger spread due to the finite angle (one half degree) the sun subtends.
The second problem is that the sharpness of the boundaries and the concentration ratio are very sensitive to the separation of the target from the lens due to the large aperture of such lens designs.
The third problem involves the typical situation that the lenses of the concentrator are square or polygon shaped and the rays outside the inscribed circle which amount to about 21% of the total square surface, cannot be mapped neatly into the typical target.